1. Field of the Invention
The present invention relates to a method and apparatus for insulating elements and, more particularly, to a method and apparatus for simultaneously insulating a plurality of electrically conductive elements to achieve a more reliably bonded flat electrical cable in a more economical manner, for facilitating exposure of the electrically conductive elements to allow electrical contact to be made, and for bonding flexible electrical circuits.
2. Description of the Prior Art
Electrical cable is simply a plurality of electrically conductive elements aligned parallel one another in a predetermined spacing encapsulated (except for the ends of the cable) with synthetic resin electrical insulative material. Physically, the cable appears as a thin flexible strip of synthetic resin having small embedded metallic wires.
Efforts to make electrical cable less expensively, quicker, and more reliable have spawn many methods and apparatus exemplified by the prior art. One such method, sometimes referred to as the hot-roll method, comprises locating two grooved rollers closely adjacent one another, heating the rollers to a temperature to cause fusion of the synthetic resin insulative material used and passing a plurality of conductive elements sandwiched between insulative strips to achieve a bonded electrical cable. Another method, such as exemplified by U.S. Pat. No. 3,082,292 to R. W. Gore, comprises moving the sandwich of electrical wire and insulator between two grooved rollers and then passing the formed but unbonded cable to an oven to achieve bonding. Yet, another method as exemplified by U.S. Pat. No. 3,531,045 to L. L. Emmel et al., commonly referred to as the chill-roll process, comprises passing the layered electrically conductive elements and insulative material through a heating device to heat the entire unbonded cable to well over the temperature necessary for bonding and then passing the cable through two grooved rollers which are at a temperature below that of bonding.
Each of the prior art processes provided an unreliable bond, that is, fusion of the abutting surfaces of the insulative material could not be reliably accomplished without causing substantial difficulties with other portions of the cable. For example, some of the methods required heating to a substantially higher temperature than necessary to cause fusion to compensate for heat loss before maximum pressure is brought to bear to cause bonding; this excessive heat frequently oxidized the electrically conductive elements thereby making future electrical contact difficult. Excessive heat creates the generation of excessive amounts of gas causing gas bubbles in the cable. Still another problem related to internal stresses formed in the insulative material during the insulative material's manufacture. When heated, the areas, representing overly stressed and normally stressed regions, expand at different rates causing wrinkles in the insulative cover. Removing wrinkles requires critical machine adjustments and excessive machine tension applied to the insulative material during the cable-making process. Upon cutting such a cable for use, the insulative material tends to recede from the conductive elements leaving the ends of the conductive elements undesirably exposed. A further problem with overheating occurs after pressure has been released; if the cable is still above fusion temperature, there is a tendency for the insulative material to deform to its original flat shape and unbond. One solution to the latter problem is to use more adhesive material to retard undesirable deformation. However, this results in a cable which is substantially thicker than need be and which concurrently increases material expenses.
Another major difficulty facing the electrical cable industry and its customers is quickly, economically and reliably exposing the electrically conductive elements of a bonded cable to allow electrical contact to be made. Present methods include grinding away the insulative material in order to expose the conductive elements; this frequently causes damage to the conductive elements since it is difficult to accurately control the grinder. Another method includes the application of heat to the insulative material to allow its separation from the conductive elements; this causes deformation of the cable, oxidizes the conductive elements and enhances the chance of cable delamination. Neither of the above methods are suited for fully automatic operation since both methods disturb the positioning of the conductive elements which is critical for automatic connection to another electrical element such as an electrical connector having multiple electrical contact positions. For example, a three inch wide cable may easily contain sixty electrically conductive wires in alignment, each wire parallel to the others. Position tolerances are .+-. 0.0005 inches. Grinding, heating or cutting through insulation which may be 0.005 inches is exceedingly delicate and difficult.
Still another problem faces the flexible circuit industry. A flexible circuit may be defined as a specifically designed layer of electrically conductive material bonded between two layers of insulative material. Insuring a reliable bond between the two insulative layers and achieving this bond in a relatively short time span has always eluded solution.